Engine Starting Electrics

To start an engine we need to rotate the crankshaft by external means which will then allow the engine run cycle to take over.

The most basic method would be to use a suitably cranked handle temporarily engaged in the end of the crankshaft and turned by some athletic individual. Better still would be the fitting of an electric motor under driver control a starter motor.

Starter Motor Engagement Functions

The most common starter motor found on modern vehicles is the pre-engaged type which uses an electro-magnet to operate switching and engagement functions, this differs from the older inertia type which requires a separate heavy current switch and engages by throwing a drive gear into engagement.

This engagement with the engine is achieved by the use of a toothed ring secured to the outer rim of the flywheel (the ring gear), when the starter is engaged drive is transmitted from the motor drive gear onto the ring gear and the engine turns over.

The Cranking Phase

For the average engine to start, a minimum speed of 100rpm is required, less than this will not produce the levels of compression and fuel vapour mixing required for proper combustion.

During the cranking phase the starter motor will demand around 150 – 200 amps from the battery, however, under extreme climatic conditions even the average family 2 litre hatchback can ask for 450  500 amps to overcome the initial resistance to rotation and friction found within the engine.

To give some idea of the torque required for this freezing cold morning scenario, the average wheel nut is tightened to around 90- 110Nm, the amount required to start the engine moving can be as much as 450 – 500Nm dropping to 160 – 200Nm to maintain the 100rpm minimum crank speed.

It must be noted that cranking should be restricted to a maximum of 10 seconds at a time, extended crank times, more than 3 seconds, will cause the motor and associated wiring to heat up leading to increased fatigue and a much shorter service life.

Starter Motor and Solenoid Functions

So how is a strarter motor controlled?

As the pre-engaged type is by far the most common the following will be restricted to this type.

The two main parts of the starter motor are the motor itself and the solenoid which is usually mounted on the top of the motor casing, the motor is devoted purely to rotating like most other electric motors, the solenoid has two functions –

  1. To handle the high current switching, and,
  2.  To push the drive gear into mesh with the ring gear.

It would be impractical to run the large high current cables from the battery to a switch mounted in the cab then down to the starter, so the pre-engaged system uses the solenoid to handle the heavy current switching required.

This is done under the control of a much lower current circuit using far smaller wires from the starter switch which can now be incorporated into a combined ignition/start switch assembly.

The second job for the solenoid is to facilitate the engagement of the starter drive gear into the engine ring gear. Both of these tasks are carried out by an electro-magnet making up the solenoid assembly.

The high current feed from the battery is connected to one of the large terminals of the solenoid; the other large terminal feeds the motor itself. In the at rest position this circuit is open (no current flow), when the solenoid is operated by a low current feed from the starter switch the centre plunger of the solenoid pulls the drive gear into mesh with the ring gear and closes the high current circuit so powering the motor.

As the engine starts, the driver releases the ignition key from the crank position thereby cutting the low current feed to the solenoid which in turn breaks the supply to the motor and brings the drive gear out of mesh assisted by a return spring.